Creating your Qt Designer plugin

In Chapter 4, Conquering the Desktop UI, we created a custom Qt widget in Qt Designer using the promoting technique. It is now time to learn how to create a custom Qt widget by building a plugin for Qt Designer. Your widget will be available from the Design mode in the Widget Box alongside other regular Qt widgets. For this project example, we will create a FilterWidget class that processes an input image to apply a filter. The widget will also display the filter name and a dynamic thumbnail of the filtered picture.

This project is composed of two sub-projects:

• filter-plugin-designer: This is a Qt Designer plugin containing FilterWidget class and the image processing code. This plugin is a dynamic library that will be used by the Qt Creator to offer our new FilterWidget in the Form Editor.

• image-filter: This is a Qt Widget application using multiple FilterWidget. The user can open an image from their hard disk, select a filter (grayscale, blur, and so on), and...